Problem: In chemistry class, the girl to boy ratio is $9$ to $3$. If there are $45$ girls, how many boys are there?
Explanation: A ratio of $9$ girls to $3$ boys means that there is a group of $9$ girls for every group of $3$ boys. If there are $45$ girls, then there are $5$ groups of $9$ girls each. Because we know the ratio is $9:3$ , there must be $5$ groups of $3$ boys each. There is a total of $15$ boys in chemistry class.